The present invention concerns digital signal phase modulation that is particularly suitable for spread spectrum applications.
Two well-known modulation techniques for radio frequency (RF) bandwidth compression (ie., promoting spectral efficiency) are quadrature amplitude modulation (QAM) and quadradrature phase shift keying (QPSK). Both of these methods suffer a loss of signal power with increasing modulation levels or the accompanying bandwidth compression. In addition, these methods are likely to result in errors because of low signal to noise ratio (SNR). To compensate for these errors, an increase in power approximately equal to to the square of the increase in bandwidth compression is required. For example, ten times bandwidth compression requires a 100 times increase in power.
Some commonly used modulation techniques such as frequency shift keying (FSK), minimum frequency shift keying (MFSK), GMSK, and QAM transmit non-return to zero (NRZ) line coded data that concentrates bandwidth around a carrier. Bi-Phase coding (eg., Manchester and Miller coding, as known) keeps the information sidebands away from the carrier. FIG. 1 illustrates the spectral characteristics of digital bi-phase signals such as QPSK, BPSK, QAM, etc. The spectrum of line coded signals concentrates around baseband, whereas for digital bi-phase signals the spectrum moves away from the origin to a point around half of the transmitted bit rate. In order to obtain significant savings in the transmitted bandwidth, only one sideband need be transmitted. A bi-phase signal is time varying and hence has no zero crossing points that vary with time. Bi-phase codes are polar and have little or no DC component.
Another modulation method, pulse width modulation, is employed at baseband for spectrum encoding and produces output pulses which are integer multiples of the clock period. With this method, phase delays to the end or center of a data bit distinguish between xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d logic levels, respectively. Miller encoding is one example of this type of modulation. Modulating an RF carrier with this type of modulation does not conserve bandwidth.
U.S. Pat. No. 4,742,532 issued to H. R. Walker describes a method of modulation referred to as variable phase shift keying (VPSK). VPSK modulation encodes changes between xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d logic states of a binary non-return to zero data signal. The encoding produces a pulse signal having varying periods of 4/4, 5/4, and 6/4 multiples of the bit period according to a predetermined set of coding rules. According to the coding rules, no change in the data polarity is coded as a 4/4 bit width, a change in data polarity is coded as a 5/4 bit width, and a conditional case coding for the last bit is coded as a 6/4 bit width period to indicate a required reset of the coding/decoding system. This method can theoretically produce a Nyquist factor of 7.2 bits per Hz-bandwidth.
U.S. Pat. No. 5,185,765 of H. R. Walker describes an improved method of VPSK modulation. With this method, each input data bit has a bit period constituted by M clock periods. Data bit polarity changes are phase shift key coded with waveform widths of M/M, M+1/M, and M+2/M bit periods, where M is an even integer greater than 3. According to the coding rules, no change in the polarity of the data is coded as M clock periods, a change in the polarity of the data is coded as M+1/M clock periods, and a final encoding bit for the period to indicate a reset occuring at the Mxe2x88x921 data bit polarity change is coded as M+2/M clock periods. This method theoretically allows a signal spectrum to fit into one-sixth the bandwidth of a baseband NRZ equivalent signal, resulting in Nyquist efficiencies of up to 15.3 bits per Hz-bandwidth for 10 modulation levels. This variable phase shift causes the coding/decoding system to lose synchronization with the beginning of each bit period. To overcome this problem, a reset bit must be embedded in the data for every bit period.
A disclosed method of coding/decoding exhibits operating advantages compared to the systems described by Walker. In particular, the disclosed variable aperture coding method provides twice the efficiency of the method described in U.S. Pat. No. 5,185,765 of Walker, and is more economical with respect to hardware and software requirements.
A variable aperture coding system according to the principles of the present invention employs the following algorithm to encode an input NRZ bitstream.
If the bitstream exhibits a phase change from a logic 0 to a logic 1, an associated coded data bit exhibits a bit width change in one direction (eg., increases) proportional to a predetermined factor N.
If the bitstream logic level remains unchanged, the bit width of an associated coded bit is maintained at a predetermined original bit width.
If the bitstream exhibits a phase change from a logic 1 to a logic 0, an associated coded bit exhibits a bit width change in a different second direction (eg., decreases) proportional to predetermined factor N.